Flash memory commonly uses Fowler-Nordheim tunneling for programming memory cells. The tunneling current is dependent on the voltage gap between the control gate and a channel region. When programming memory cells, a program voltage is applied to a wordline containing memory cells being programmed and other cells which are to be inhibited from programming. Program disturb occurs when programming of memory cells causes an inhibited memory cell on the same wordline to become inadvertently programmed.
In NAND flash, a program-inhibit scheme commonly referred to as self boosting may be utilized to enhance channel level and source/drain region in cells that are to be inhibited from programming. A seed voltage is placed on the inhibited bitlines to provide an initial voltage to the channel/source/drain region. Self boost applies elevated voltage to the deselected wordlines to increase the voltage in the channel/source/drain region above the initial voltage, using capacitive coupling between the wordlines and the channel/source/drain. The efficiency of the coupling depends on the boosting ratio (ratio of the capacitance between wordline and channel/source/drain to the capacitance between wordline and the bottom of the depletion region below the channel/source/drain). As NAND technology scaling proceeds, the boosting ratio decreases due to the increase of parasitic capacitance.